In some chemical processes with parallel-competing reactions or consecutive-competing reactions, the reaction products or intermediate may further react with raw material component(s), which produces undesirable by-products or impurities. In this case, the initial mixing of reactants has a significant impact on the yields and selectivity of the target products, i.e. the distribution of the products, especially when the mixing rate of the reactants is less than or similar to the chemical reaction rates. Meanwhile some other indexes of the process, such as the energy consumption, will be affected by the mixing efficiency of the reactants. So the reactor with fast mixing efficiency should be considered in the design of the whole process. For example, in the production of isocyanates (MDI or TDI) by phosgenation of amines, the process mainly includes so-called cold phosgenation stage and hot phosgenation stage. At the cold phosgenation stage, mono- or polyamines and phosgene are dissolved in an inert solvent, respectively, such as chlorobenzene, dichlorobenzene, toluene, chlorinated naphthalene, or 1,2,4-trichlorobenzene, etc., and then reacted at a lower temperature in the range of 0-90° C. At this stage, main products or intermediate products include carbamyl chloride (RNHCOCl), amine hydrochloride (RNH2.HCl), and a small amount of urea. The main reactions are as follows:RNH2+COCl2→RNHCOCl+HCl  (1)RNH2+HCl→RNH2.HCl  (2)RNH2+RNHCOCl→RNCO+RNH2.HCl  (3)RNH2+RNCO→RNHCONHR  (4)
At the cold phosgenation stage, the mono- or polyamines firstly react with phosgene, as shown in reaction (1), producing carbamyl chloride and hydrochloric acid. Reaction (1) is an exothermic reaction with fast reaction rates. Meanwhile, HCl produced from reaction (1) conducts a fast reaction (2) with the mono- or polyamines giving amines hydrochloride. The carbamyl chloride and amine hydrochloride are insoluble substances in the solvent and appears in the solid formulation. When the micro-mixing efficiency of phosgene and amines is lower, that is, the amines is excessive to the phosgene in some zone of the reactor, the excessive amines will react with carbamyl chloride or isocyanates through reactions (3) and (4), producing the byproduct of urea which is a sticky substance insoluble in the reaction system and can plug the reactor. This process involves complicated consecutive-competing reaction, in which the main reaction occurs instantaneously with reaction times of several milliseconds or less, and its products can further conduct a fast reaction with the raw material reactants to produce the insoluble byproducts. Therefore, the initial mixing efficiency of the two reactants will have great effect on the yield and selectivity of the target product. So it is necessary to design a mixer-reactor which can achieve the fast mixing of liquid reactants, increase the yields and selectivity of the target products, and reduce the yield of the sticky byproduct.
Fast mixing of fluid could be facilitated by the impingement of two cross-flow streams. One example is, one fluid stream is injected into another fluid stream crossly through a plurality of small holes. So the stream through the holes is split into many small streams which are injected into the other stream and then surrounded by the main stream rapidly, giving the fast mixing of the two streams.
U.S. Pat. No. 3,226,410 discloses a process for the manufacture of isocyanates, wherein the mixer-reactor for the phosgenation of amines is a hole-jetting type tubular reactor (as shown in FIG. 1), in which amines solutions are injected into phosgene streams crossly through small holes on the pipe wall so that fast mixing of the reactants could be achieved. However, the detailed structure of the tubular reactor has not been described in the patent. It only covers the range of the flowing parameters of the two streams, viz. the Reynolds number that is not less than 2100. Furthermore, since the mixer-reactor disclosed in the patent can not achieve the fast mixing of the reactants in large scale, more solvent and phosgene should be used to reduce the undesirable byproducts, which would increase the energy consumption for the recovering of the solvent and phosgene.
Bayer's U.S. Pat. No. 5,117,048 discloses another hole-jetting type mixer-reactor (as shown in FIG. 2) for the production of isocyanates, in which one fluid stream (mono- or polyamines) is injected into the other stream (phosgene) crossly through the holes arranged on the neck of the reactor at the same horizontal line to achieve the fast mixing of two reactant streams. This good mixing efficiency benefits from the design of the neck part of the reactor and cross-flow impingement of the streams, which enhance the turbulence of the mixing streams and quicken the mixing process. By using the disclosed mixer-reactor, the mole ratio of solvent and phosgene to amines could be reduced significantly.
It can be seen from the above description that good mixing efficiency could be achieved by injecting one fluid stream into another stream in cross-flow manner through a plurality of holes arranged around a pipe. However, the injected stream can only reach a certain depth of the main fluid stream because of the resistance of the fluids, i.e. excellent mixing efficiency can be achieved only in the case of small pipe, which restrict the scale of the plant. In detail, the injected stream from the holes could not reach the center of the mixing pipe immediately when the diameter of the mixing pipe is large enough. So it will take long distance or long time for the joined stream to mix into the main stream when the scale of the plant is large. Therefore, there is a demand for the design of the mixer-reactor with excellent mixing performance for the initial mixing of the reactants in large-scale plant.